1. Field of the Invention
This invention relates to an apparatus and method for generating pulses and, particularly, generating pulses indicating the rotational movement and position of a shaft.
2. Description of the Prior Art
It is known in automotive vehicles to have a rotating wheel provide signals representative of the rotation of a crankshaft of an engine. In particular, the wheel can have a number of radially protruding teeth which cause signals to be produced each time they pass a detector. Thus, determining the number of teeth which pass the detector during a given time period can be used to determine the revolutions per minute of the crankshaft. Further, the occurrence of a pulse generated by a tooth indicates that the rotational position of the wheel, and thus the crankshaft, is at one of those positions indicated by the teeth on the wheel.
Signals can also be generated which characterize one particular angular reference position of the crankshaft. Such signals are necessary, for example, to initiate ignition. Although each one of the teeth on the wheel may indicate that an ignition should take place, the reference position is necessary to indicate when the sequence of cylinder firing should start. To generate a reference signal, it is known to make use of an additional wheel having a single protruding tooth and an additional detecting device for detecting the single tooth. Thus, a signal indicating a particular reference position occurs when the receiver detects passing of the single tooth. Such an arrangement is undesirable because there is the additional expense of the assembly of the wheel having a single tooth and the detector. The positioning of the wheel with the single tooth with respect to the other wheel must be done carefully so that there is an accurate angular relationship between the two wheels.
The prior art also teaches attempts at avoiding the necessity for two separate wheels and the attendant disadvantages. In a wheel having a plurality of teeth, one tooth is longitudinally cut to provide two tooth positions with a gap therebetween. If the spacing between adjacent uncut teeth is equal to the width of a tooth, the presence of the cut tooth with the gap can be detected by comparing the duration of the separation between adjacent teeth and the width of the immediately preceding tooth. That is, the spacing separating a cut tooth and adjacent tooth is greater than the width of one of the cut tooth portions.
However, such a scheme also has disadvantages in that the width of the tooth must be sufficiently wide to be cut into two portions and still provide a signal. Thus, the minimum width of the tooth is determined by the ability of a manufacturing process to form two tooth portions out of a single tooth. Further, the counting technique used to determine the presence of a cut tooth is based upon a spacing between adjacent teeth equal to the width of each tooth. Thus, such a system may not be desirable where it is necessary to have relatively narrow pulses which are spaced further apart than the pulse width. Further, relatively elaborate circuitry is required to determine the occurrence of a split tooth. Still further, it would be desirable to be able to establish the position of the reference point without the need for detecting or recognizing any of the other positions denoting angular position.
A further problem which has existed without a completely satisfactory solution is to provide a sufficiently accurate indication of revolutions per minute when only two angular positions of the wheel need be known to determine firing of the cylinders. For example, in a four cylinder engine, two cylinders are fired during each rotation of the wheel. Thus, although only two positions are required on the wheel, the variance in determining the revolutions per minute is sufficient that it is desirable to have additional reference positions on the wheel for more frequent and thus more accurate computation of crankshaft revolutions per minute. One problem has been that these additional reference positions create erroneous spark plug firings. While the signals designating the different reference positions should be different, they should be sufficiently alike in such parameters as magnitude that the apparatus used to detect one signal can easily and efficiently detect the other signal.
In addition to problems associated with determining an advantageous shape for the signal initiating wheel, producing the electrical signal itself has presented problems. Known techniques include the use of Hall effect sensors and wound coils which produce an electric current in response to a changing magnetic field. Typical disadvantages associated with known sensors include insufficient sensitivity and undesirable response to spurious signals. These are some of the problems this invention overcomes.